PTC thermistors have been used as the components to protect a device against an overcurrent. Exposure to an overcurrent in an electric circuit causes the conductive polymer having the PTC characteristics used in a PTC thermistor to heat up and expand by self heating. The thermal expansion increases resistance of the conductive polymer sheet in the PTC thermistor, and thus reduces the current to a safer level.
A conventional chip PTC thermistor is described below.
One known chip PTC thermistor is disclosed in Japanese Laid-open Patent No. H9-503097. The chip PTC thermistor is formed of a resistive material having the PTC characteristics, the chip thermistor having a first surface and a second surface. The chip thermistor comprises a PTC resistor element that specifies a space between the first surface and the second surface, a lateral conductive member provided within said space through the first surface and the second surface of PTC element, the conductive member being fixed to said PTC element, and a first layered conductive member connected physically and electrically to the lateral conductive member. FIG. 14A shows a cross sectional view of the conventional chip PTC thermistor, and FIG. 14B is the plan view. In FIGS. 14A and B, a resistor body 61 is formed of a conductive polymer having PTC characteristics, electrodes 62a, 62b, 62c, 62d are formed of a metal foil, conductive members 64a, 64b are formed inside the openings 63a, 63b by plating, and electrically couple the electrode 62a with 62d, and the electrode 62b with 62c respectively.
A method for manufacturing the conventional chip PTC thermistor is described below. FIGS. 15A–15D, and FIGS. 16A–16C illustrate the process steps for manufacturing the conventional chip PTC thermistor.
Polyethylene and conductive carbon particles are mixed to form a sheet 71 shown in FIG. 15A. The sheet 71 is sandwiched by two sheets of a metal foil 72, as shown in FIG. 15B, and these are heat pressed together to be integrated into a sheet 73 as shown in FIG. 15C. After undergoing electron beam irradiation, the integrated sheet 73 is provided with through holes 74 in a regular pattern arrangement as shown in FIG. 15D, and then a metal film 75 is formed by plating to cover the inner surface of the through hole 74 and the metal foil 72, as shown in FIG. 16A. Then, as shown in FIG. 16B, an etched slit 76 is formed in the metal foil through a photo-lithographic process. And then, it is cut off along a longitudinal cut line 77 and a lateral cut line 78 to be separated into piece chips to obtain the conventional chip PTC thermistor 79 as shown in FIG. 16C.
In the conventional chip PTC thermistor of the above configuration, however, the two electrodes 62a and 62b, or 62c and 62d, which are to be connected with a printed circuit board when the chip thermistors are mounted thereon, are disposed on only one surface of the chip thermistor (ref. FIG. 14A). As a result, when the chip thermistors are mounted on a printed circuit board and reflow-soldered, solder fillets formed by the soldering are not visible from above because they are shadowed by the chip thermistors. Therefore, it is difficult to make sure of the state of soldering by visually inspecting the soldered portion. Furthermore, because the electrodes of the chip thermistors are not disposed at their sides, the flow soldering process is not applicable.
Furthermore, in the above described conventional manufacturing method, dislocation of the cut lines in relation to the location of a through hole is not avoidable because of dispersions in the accuracy of the sheet aligning and the cutting operations. This readily leads to a variation in the area of coupling between the conductive member formed within the through hole and the top/bottom electrodes. FIG. 17A shows a state wherein no dislocation exists between the through hole and the cut line, while FIG. 17B shows a state where there is a dislocation. In FIGS. 17A and 17B, numeral 81 denotes a through hole, 82 is a cut line, 83 is an electrode, 84 is an etched slit. In a case where a part of one through hole 81, among the through holes located at both sides of a cut line, is cut as a result of the above described dislocation, as shown in FIG. 17B, the area at a contact section 85 making contact between the conductive member disposed within the through hole and the top/bottom electrodes becomes smaller, as compared with a case where there is no such dislocation. The case caused by a dislocated cut line is illustrated in FIG. 17C. A problem with the reduced contact area between the conductive member and the top/bottom electrodes is that the junction between the conductive member and the top/bottom electrodes is easily cracked due to stress caused thereon by repetitive expansion and shrinkage of the conductive polymer.
The present invention addresses the above problems and aims to provide a chip PTC thermistor, as well as a method of manufacturing the same, wherein the soldered portion can be inspected easily visually after the chip thermistors are mounted on a printed circuit board, and the chip PTC thermistor can be soldered by flow soldering. Furthermore, the coupling between the conductive member and the electrodes has only a small dispersion in the strength of connection against the stress that caused as a result of expansion and shrinkage of the conductive polymer.